If the motive behind the plane's disappearance is indeed criminal, it begs the question: Why can these systems be turned off at all?

The answer is simple — for safety reasons.

ACARS allows pilots and those on the ground to swap messages about things like air traffic and weather reports.

A pilot can shut the whole thing down by disconnecting a circuit breaker, and the plane's manual would tell him how to do it. That's for good reason, Patrick Smith, an airline pilot and author of Cockpit Confidential, wrote on his blog:

"In the interest of safety — namely, fire and electrical system protection — it's important to have the ability to isolate a piece of equipment, either by a standard switch or, if need be, through a circuit breaker," Smith explained.

Without going that far, he could disable the plane's ability to send messages, Michael G. Fortune, a retired pilot who now works as an aviation consultant and expert witness, said in an interview. "The average guy would have to look around" to know how to do it, but it would take only a few minutes to figure it out. The plane could still receive messages, though.

The second communication system on the missing Boeing 777 is the transponder, used mostly to talk with air traffic control when the plane is near or at the airport.

When the Malaysia plane disappeared, it was over the ocean, so the transponder wasn't useful anyway — it was well out of range of the ground radar systems it uses to communicate. But if someone brought the plane back to land and wanted to keep it hidden, he would have to disable the transponder.

Easy.

"Generally, you do always want it on," Doug Moss, a former test pilot and aircraft accident investigator, told Business Insider. But that's not the case at airports with surface radar movement equipment, which gives air traffic controllers the precise position of aircraft on the ground. At those airports, data from the plane's transponder is unnecessary and can make things confusing. It "clutters up the radar screen for the ground controllers," Moss said.

So there's an off switch, kind of. On the 777-200ER planes Fortune flew for Continental, he said the transponder could be switched to "standby," which for the most part disabled it.

Plus, the transponder is usually switched "off" once the plane is parked, since there's no more need for it. "You don't want to interfere with airborne aircraft," Fortune said.

So making Flight 370 "disappear" would have been relatively straightforward for a trained pilot with his manual handy.